The calculation of the optical and electronic properties of semiconductor nanopstructures is still based for the most part on highly approximated, continuum-like models such as the effective-mass approximation. These models do not take into account the atomistic structure of the quantum dots, thus missing crucial features that become essential when the system is only a few nanometers in size. We have recently developed an atomistic pseudopotential approach to the calculation of excited states in semiconductor nanostructures. This approach involves two steps: (i) The electronic potential is expanded as a superposition of screened atomic pseudopotentials, which are fitted to experimental bulk transition energies, effective masses, and deformation potentials, and to first-principles bulk electronic wave functions. The single-particle Schroedinger equation for the nanostructure is then solved using powerful, O(N) methods. (ii) The electronic excited states (such as excitons, multi-excitons, etc.) are then calculated by solving the many-particle Scroedinger equation in a basis set of Slater determinants, obtained by promoting one or more electrons from the valence band to the conduction band. I will discuss applications of the pseudopotential method to predict the optical absorption and emission spectra of charged excitons, bi-excitons and tri-excitons in CdSe colloidal quantum dots.